The invention relates to sol-gel structure fabrication and more particularly to monitoring of cracks during a sol-gel tube fabrication process.
Sol-gel tubes are formed as precursors to optical fibers. As demand for optical fiber continues to escalate, improved diagnostic and process monitoring tools are desirable to increase sol-gel tube manufacturing yield. In particular, it is advantageous to monitor processes for sol-gel tube cracking.
Indirect measurement of process conditions such as temperature, humidity and gas flow as is currently done, often does not provide accurate correlation of such conditions with tube cracking.
Existing scanning and imaging techniques have limited value in detecting cracks. Scanning methods such as laser ultrasonic techniques, and spectroscopic methods such as infrared and UV-visible techniques are slow and difficult to implement. Imaging of tubes to monitor cracking using a camera requires highly sophisticated imaging software.
Acoustic emission monitoring has been used to detect large scale movement such as in security system motion sensors. Detection of small scale material movement, such as occurs during sol-gel tube cracking, by acoustic emission monitoring is not presently employed.
Because acoustic emission from cracking arises from unpredictable locations at random times and with an unanticipated waveform involving a relatively broadband of frequencies, it is much more difficult to analyze than conventional well-characterized ultrasonic pulses commonly used in nondestructive testing. Despite innumerable measurements of acoustic emission signals from a variety of sources, their waveforms and frequency spectra, the prediction of a failure mode based on information provided by acoustic emission, has proven difficult in the past.
Sol-gel formation process temperatures may reach higher than 1000xc2x0 C. These high temperatures limit in situ monitoring of sol-gel tubes during the formation process. This is problematic because the sol-gel tube formation process typically spans a 48 hour period, making it difficult to isolate a process step or steps during which problems such as cracking occur.
Accordingly, there is a need for improved crack detection in sol-gel tubes.
Embodiments of the invention provide silica structure crack detection methods and apparatuses particularly useful in sol-gel fabrication processes. A wave signature of a crack in the silica structure is sensed to indicate that cracking has occurred. Sensing may be by active or passive techniques and may include contact and non-contact methods of monitoring.
Further disclosed is a silica structure fabrication process development method wherein cracks are monitored by sensing a wave signature of a crack to isolate a process step or steps in which cracking has occurred. Process parameters are then adjusted in the isolated step to diminish or eliminate cracking.